Unfortunately, C99 introduced more keywords, but not
enough to total 64. Worse still, some of them have awkward
names. The new keywords are: inline
restrict _Bool _Complex _Imaginary.

What keyword is never
used?

The keyword auto. Within
functions, auto variables are local to the stack frame, in
contrast to static variables. But since this is the default,
auto is useless.

One could argue there are other redundant keywords: for
example, one can always replace an if statement with the
ternary operator, or for loops with while loops, but we can
lose clarity doing so. The auto
keyword may be dropped with no loss.

In GNU C, auto is used to
forward-declare a nested function.

What keyword was
withdrawn?

Originally, entry was
reserved. There were plans to support multiple entry points
to a function that were ultimately abandoned.

How many keywords can you fit in a variable
declaration?

If the variable is a struct or union we can have an
arbitrary number of members. Pointers can also be extended to
any desired length: